Nighttime visibility of objects and people is a continuing problem, particularly for the operators of vehicles such as trucks and automobiles. Approaches to increasing their nighttime visibility can be either active or passive. Active systems provide conspicuity by providing a steady light source, flashing light source, or a combination of steady and flashing light sources. Although active systems do provide conspicuity, they must be provided with energy, typically electrical energy, to provide the desired light. Energy sources are not always available or they may be depleted such that the light sources will not operate. As a result, active systems find limited application for providing long term conspicuity.
Examples of passive systems include diffuse reflectors, specular reflectors, and/or retroreflectors. Retroreflectors can return a significant portion of incident light, that would otherwise be reflected elsewhere, back towards a light source, such as the headlights of a car or truck. Retroreflectors are typically constructed of beads (see, e.g., U.S. Pat. Nos. 4,025,159 to McGrath; 4,983,436 to Bailey et al.; and 5,066,098 to Kult et al.) or they can include cube corner elements (see, e.g., U.S. Pat. Nos. 5,272,562 to Coderre and 5,450,235 to Smith et al.). With many retroreflectors, the amount of returned light can make the retroreflective article appear as though it has a light source of its own when, in reality, at least a portion of the light directed at the retroreflector from the light source is merely being returned towards the source. Although retroreflectors return a significant portion of light, the incident light is typically from a steady source such as headlights. Steady incident light yields generally steady returned light from the retroreflectors, that is, the intensity of the returned light does not appreciably vary.
Investigators have made various strides towards improving the conspicuity of retroreflective articles. For example, Shusta et al. in PCT publications WO 97/41465 and 97/41464 (U.S. patent application Ser. Nos. 08/640,326 and 08/640,383) describe a retroreflective article that glitters when exposed to light. Fluorescent dyes have also been used to improve conspicuity--see for example, U.S. Pat. Nos. 5,387,458 and 3,830,682. U.S. Pat. No. 4,726,134 describes a retroreflective sign which has areas that vary in retroreflectivity to improve the sign's conspicuity.